The present invention relates generally to filter circuit constructions, and, more particularly, to a low-pass filter circuit for filtering high-frequency harmonic components of a radio-frequency signal generated by a transmitter.
A communication system is operative to transmit information (referred to hereinbelow as an "information signal") between two or more locations, and includes a transmitter and a receiver interconnected by a transmission channel. Information is transmitted by the transmitter to the receiver upon the transmission channel. A radio communication system is a communication system in which the transmission channel comprises a radio-frequency channel wherein the radio-frequency channel is defined by a range of frequencies of the electromagnetic spectrum.
The transmitter forming a portion of a radio communication system includes circuitry for converting the information signal which is to be transmitted into a form suitable for transmission thereof upon the radio-frequency channel. Such circuitry is referred to as modulation circuitry which performs a process referred to as modulation. In such a process, the information signal is impressed upon a radio-frequency electromagnetic wave. The radio-frequency electromagnetic wave is of a frequency within the range of frequencies defining the radio-frequency channel upon which the information signal is to be transmitted. The radio-frequency electromagnetic wave is commonly referred to as "the carrier signal," and the radio-frequency electromagnetic wave, once modulated by the information signal, is commonly referred to as the modulated signal.
Various modulation schemes are known for impressing the information signal upon the carrier signal to form the modulated signal. For instance, amplitude modulation, frequency modulation, and phase modulation are all modulation schemes by which an information signal may be impressed upon a carrier wave to form a modulated signal.
Radio communication systems are advantageous in that no physical interconnection is required between the transmitter and the receiver; once the information signal is modulated to form the modulated signal, the modulated signal may be transmitted over large distances.
A cellular, communication system is one type of radio communication system. Radio transceivers (conventionally referred to as radiotelephones) operative in such a cellular, communication system contain circuitry permitting simultaneous generation and reception of modulated signals to permit thereby two-way communication between a radiotelephone and remotely-located transceivers. The remotely-located transceivers, referred to as "base stations," are physically connected to a conventional telephonic network to permit communication between a radiotelephone and a fixed location of the conventional telephonic network. A cellular, communication system is formed by positioning numerous base stations at spaced-apart locations throughout a geographical area. Each base station contains circuitry to receive modulated signals transmitted thereto by one, or many, radiotelephones, and to transmit moduclated signals to the one, or many, radiotelephones.
A frequency band of the electromagnetic frequency spectrum (in the United States extending between 800 MHz and 900 MHz) is allocated for radiotelephone communication upon a cellular, communication system. The allocated frequency band is further divided into a plurality of transmission channels of defined bandwidths. Modulated signals generated by radiotelephones are transmitted upon selected ones of the transmission channels defined upon the frequency band allocated for radiotelephone communications. Similarly, modulated signals generated by the base stations are also transmitted upon selected ones of the transmission channels defined upon such frequency band.
As a byproduct of the modulation process to form the modulated signal, harmonics of the desired signal are also generated. These harmonics are essentially duplicates of the desired signal, but are of frequencies which are multiples of the desired signal. As a result, the modulated signal actually generated is comprised of spectral components of not only the desired frequencies, but also of frequencies which are multiples of the desired signal.
By way of an example, a radiotelephone operative to generate a modulated signal for transmission upon a transmission channel defined within the frequency band allocated for such communication generates a modulated signal wiich is comprised not only of spectral components forming the desired signal (of frequencies corresponding to the frequencies defining the transmission channel) but also of spectral components of harmonics of the desired signal. The spectral components of the harmonics may be of frequencies up to, and in excess of, 10 gigahertz.
Transmission of such spectral components can, of course, interfere with modulated signals transmitted at other higher frequencies.
Accordingly, radiotelephones typically include filter circuitry to attempt to attenuate transmission of all but the desired spectral components of the modulated signal.
Ceramic block filters, containing distributed circuit elements, oftentimes form the filter circuitry to attempt to attenuate transmission of all such spectral components of the modulated signal but the desired spectral components. However, due to the construction of such ceramic block filters, spectral components of the modulated signal applied thereto of frequencies corresponding to odd harmonics of frequencies of the pass bands of such ceramic block filters (such pass bands correspond, of course, to the range of frequencies comprising the spectral components of the desired signal) are not attenuated.
To attempt to prevent transmission of such odd-harmonics of the modulated signal, therefore, transmitter circuitry of such radiotelephones frequently further include low pass filters cascaded with the ceramic block filters. The low pass filters are utilized to attempt to attenuate all spectral components of the modulated signal beyond the cut off frequencies of such low pass filters.
In many instances, the low pass filters are constructed of lumped elements--i.e., discrete elements. Such lumped-element, low pass filters are useful in the attenuation of spectral components of a modulated signal applied thereto of frequencies up to several gigahertz (GHz). However, parasitic effects associated with the discrete elements of such lumped-element, low pass filters limit the usefulness of such filters to attenuate spectral components of frequencies in excess of several gigahertz. In fact, at higher frequencies, such parasitic effects result in such deteriorated performance of the low pass filters that little, or no, attenuation of the higher-frequency spectral components of the modulated signal applied thereto occurs.
Low pass filters comprised of distributed elements, i.e., elements comprised of transmission lines, are similarly also oftentimes utilized to form portions of transmitter circuitry of a radiotelephone. Such constructions of low pass filters (as contrasted to lumped-element, low pass filters) effectively attenuate the higher-frequency spectral components of a modulated signal applied thereto. However, the physical dimensions of the transmission lines forming the elements of such low pass filter construction become increasingly significant as the cut off frequency required of the low pass filter is reduced. That is to say, the physical dimensions required of the transmission lines forming the distributed elements of such low pass filters is significantly greater when the cut off frequency required of the low pass filter is reduced in frequency, for example by reducing the cut off frequency to one GHz from four GHz, the physical dimensions required of such a filter is significantly reduced. Additionally, such filters comprised of distributed elements do not effectively attenuate high frequencies. However, when the inductive and capacitive values of the components of such filters are of large enough values, the frequencies not attenuated are of such significant values to pose little problem.
As size minimization is a critical design goal in the design of portable radiotelephones (as well as other radiotelephone constructions), a low pass filter circuit constructed only of distributed elements and operative to attenuate all of the undesired spectral components of the modulated signal is unduly large.
As the portable radiotelephones (and also, radiotelephones of other constructions) are packaged in increasingly miniaturized housings, the circuitry of such radiotelephones must similarly be of reduced dimensions. The significant dimensional requirements required of a distributed-element, low pass filter as the cut off frequency of the filter is reduced can become a serious design constraint.
It is further noted that, to minimize the area required to form circuitry comprised of such distributed elements upon a surface of a circuit board (a distributed element printed upon the surface of a circuit board is referred to as a microstrip), such distributed elements may be disposed beneath the surface of the circuit board, and, for example, form a middle layer of a three-layer circuit board (a distributed element disposed beneath the surface of a circuit board is referred to as stripline).
Distributed elements forming microstrips printed upon the surface of a circuit board must be shielded to prevent electromagnetic emanations generated thereat from interfering with other circuitry of the radiotelephone. Distributed elements forming striplines disposed beneath the surface of the circuit, while not required to be shielded, can not be tuned once formed.
To attenuate all but the desired spectral components of a modulated signal generated by the transmitter circuitry of several radiotelephone constructions, in addition to the aforementioned ceramic block filters, two, or more, low pass filters are cascaded in a series arrangement. A first low pass filter comprised of lumped elements is utilized to cause attenuation of a first, and low-frequency, group of spectral components of a modulated signal applied thereto. And, a second low pass filter, comprised of distributed elements, is utilized to attenuate spectral components of a modulated signal applied thereto of a second, and higher-frequency, group of spectral components. While such a cascaded arrangement of filters attenuates the non-desired spectral components of the modulated signal, such cascaded arrangement causes additional insertion loss of the transmitted signal, as well as increased dimensional requirements due to the construction of two separate filter circuits.
What is needed, therefore, is a low pass filter construction which attenuates all spectral component portions of a modulated signal, except for desired spectral component portions of the modulated signal, but which also minimizes the dimensional requirements of the filter construction.